At its inception radio telephony was designed, and used for, voice communications. As the consumer electronics industry continued to mature, and the capabilities of processors increased, more devices became available for use that allowed the wireless transfer of data between devices and more applications became available that operated based on such transferred data. Of particular note are the Internet and local area networks (LANs). These two innovations allowed multiple users and multiple devices to communicate and exchange data between different devices and device types. With the advent of these devices and capabilities, users (both business and residential) found the need to transmit data, as well as voice, from mobile locations.
The infrastructure and networks which support this voice and data transfer have likewise evolved. Limited data applications, such as text messaging, were introduced into the so-called “2G” systems, such as the Global System for Mobile (GSM) communications. Packet data over radio communication systems became more usable in GSM with the addition of the General Packet Radio Services (GPRS). 3G systems and, then, even higher bandwidth radio communications introduced by Universal Terrestrial Radio Access (UTRA) standards made applications like surfing the web more easily accessible to millions of users (and with more tolerable delay).
Even as new network designs are rolled out by network manufacturers, future systems which provide greater data throughputs to end user devices are under discussion and development. For example, the so-called 3GPP Long Term Evolution (LTE) standardization project is intended to provide a technical basis for radiocommunications in the decades to come. Among other things of note with regard to LTE systems is that they will provide for downlink communications (i.e., the transmission direction from the network to the mobile terminal) using orthogonal frequency division multiplexing (OFDM) as a transmission format and will provide for uplink communications (i.e., the transmission direction from the mobile terminal to the network) using single carrier frequency division multiple access (SC-FDMA).
Modern wireless communication systems targeted for packet-based communication often include hybrid ARQ (HARQ) functionality on the physical layer to achieve robustness against the impairments of the radio channel. LTE and Wideband Code Division Multiple Access (WCDMA) are two examples of systems in which such functionality is available. The basic idea behind HARQ is to combine forward error correction (FEC) with ARQ by encoding the information containing data block and then adding error-detection information such as CRC. After reception of the coded data block, it is decoded and the error-detection mechanism is used to check whether the decoding was successful or not. If the data block was received without error, an ACK message is sent to the transmitter indicating successful transmission of the data block and that the receiver is ready for a new data block. On the other hand, if the data block was not decoded correctly, a NACK message is sent meaning that the receiver expects a retransmission of the same data block. Subsequent to the reception of the retransmission, the receiver may choose to either decode it independently or utilize some or all previous receptions of the same data block in the decoding process.
The channel encoded bits originating from the same block of information bits is typically referred to as a “codeword”. This is also the terminology used in the LTE specifications to describe the output intended for a particular subframe from a single HARQ process serving a particular transport block and is the result of processing the information bits by performing, for example, turbo encoding, rate matching, interleaving, etc. Another interesting feature of LTE is its support for multiple antennas at both the transmit side and the receive side. In a multiple transmit antenna device or system, the resulting codewords are then modulated and distributed over the transmit antennas for transmission. The first modulated codeword may, for example, be mapped to the first two transmit antennas and the second, modulated codeword may be mapped to the two remaining transmit antennas in a four transmit antenna system.
Precoding is a popular technique used in conjunction with multi-antenna transmission. The basic principle involved in precoding is to mix and distribute the modulation symbols over the antennas while potentially also taking the current channel conditions into account. Precoding can be implemented by, for example, multiplying the information carrying symbol vector containing modulation symbols by a matrix which is selected to match the channel. Sequences of symbol vectors thus form a set of parallel symbol streams and each such symbol stream is typically referred to as a “layer”. Thus, depending on the choice of precoder in a particular implementation, a layer may directly correspond to a certain antenna or a layer may, via the precoder mapping, be distributed onto several antennas (also known as antenna ports). The mechanism by which codewords are assigned to particular layers in such systems is referred to as “mapping” or, more specifically, as “codeword to layer mapping”.
In a multi-antenna system (often referred to as a MIMO system), it may be useful to transmit data from several HARQ processes at once, which overall process is also known as multi-codeword transmission. Since the codewords are mapped to layers, the process may alternatively be referred to as multi-layer transmission. Depending on the radio channel conditions, this process can substantially increase the data rates, since in favorable conditions the radio channel can roughly support as many layers as the minimum of the number of transmit and receive antennas. This means that the channel can at most support the simultaneous transmission of a certain number of codewords, and that particular number in turn depends on the codeword to layer mapping. In the simplest case, each codeword maps to a single layer and then the number of supportable layers obviously equals the number of supportable codewords. One of the most significant characteristics associated with the channel conditions in the field of high rate, multi-antenna transmission is the so-called channel rank. The channel rank can vary from one up to the minimum number of transmit and receive antennas. Taking a 4×2 system as an example, i.e., a system or device with four transmit antennas and two receive antennas, the maximum channel rank is two. The channel rank varies in time as the fast fading alters the channel coefficients. Roughly speaking, the channel rank also determines how many layers, and ultimately also how many codewords, can be successfully transmitted simultaneously. Hence, if for example the channel rank is one at the instant of transmission of two codewords which are mapped to two separate layers, then there is a strong likelihood that the two signals corresponding to the codewords will interfere so much that both of the codewords will be erroneously detected at the receiver. The number of layers per channel use (in e.g. LTE a channel use would correspond to a single resource element) that are simultaneously transmitted is sometimes referred to as the transmission rank. With pure spatial precoding schemes such as the spatial multiplexing mode in LTE, the transmission rank equals the number of layers.
In conjunction with precoding, adapting the transmission to the channel rank involves using as many layers as the channel rank. In the simplest of cases, each layer would correspond to a particular antenna. Taking, purely as an example, the current the four transmit antenna case in LTE systems, the maximum number of codewords is limited to two while up to four layers can be transmitted. For devices or systems which have only two transmit antennas, the mapping is relatively straightforward since the number of layers equals the number of codewords. However, for devices and systems having, for example, four or more transmit antennas, there are potentially fewer codewords than layers, so the codewords need to then be mapped to the layers in some predetermined way. The issue then arises regarding how to map the codewords to the layers. Various conventional mappings from codewords to layers have been proposed and will be described in more detail below. Although these conventional mappings work well when considering, for example, first time transmission performance, they may not be optimal under other circumstances, e.g., when considering the efficiency of HARQ operation for retransmissions.
Accordingly, it would be desirable to provide other codeword to layer mappings for systems, methods, devices and software which avoid the afore-described problems and drawbacks.